1. Field of the Invention
The present invention relates to an actuator to operate a frictional engaging device such as a frictional clutch, frictional brake, etc., and in particular relates to an actuator which performs one function by selectively causing one of two opposing motions developed from each of two frictional engaging devices, such as actuators for a forward/reverse switching device or a high/low mode switching device. In detail, the present invention relates to an actuator for a frictional engaging device which mechanically converts rotational power transmitted from a power source such as a motor into a thrust force.
2. Description of the Related Art
In general, a power transmission device for an automobile, etc., such as a continuously variable transmission (The Japanese patent publication No. 63-158353; unpublished at the application of the present invention) which is constitued by the combination of a continuously variable transmission device (CVT) using a metal belt and a high/low mode switching device and a forward/reverse switching device, has devices which have certain functions which operate as a result of one or the other of two opposing motions of two frictional engaging devices. For example, these devices include a forward/reverse switching device which engages certain element of a planetary gear mechanism and at the same time releases another element so that the forward/reverse switching device performs a certain function, and a high/low mode switching device which connects a certain element of the planetary gear mechanism with an input shaft, etc. to perform another function and restrains another element to perform another function. Each element of the planetary gear mechanism is controlled by frictional engaging devices such as multiple disc clutches and multiple (or band) brakes, etc.
For example, a planetary gear mechanism for the high/low mode switching device has a low coast & reverse brake which operates the planetary gear mechanism as a reduction mechanism (namely in a low speed mode) and a high clutch which works as a split drive mechanism (namely in a high speed mode), while a planetary gear mechanism for the forward/reverse switching device has a forward clutch which takes up normal rotation and a reverse brake which takes up reverse rotation.
Conventionally, these frinctional engaging devices employ a hydraulic actuator (having a hydraulic-thrust conversion mechanism) composed of a piston and a cylinder to press or release an operation plate and a receiving plate.
In the case where the above hydraulic actuator mechanism is employed, an oil pump is driven by a power source such as an engine to produce hydraulic pressure. The hydraulic pressure developed by the oil pump is adjusted by a hydraulic control mechanism such as a regulator valve, the adjusted hydraulic pressure is switched by a hydraulic switching mechanism such as a control valve, and the hydraulic pressure from the hydraulic switching mechanism is supplied to a hydraulic-thrust conversion mechanism composed of a hydraulic actuator. The hydraulic pressure is converted into a thrust force by the hydraulic-thrust conversion mechanism, the thrust force is applied to a frictional engaging device such as a multiple disc clutch, etc. through a pressing mechanism for frictional engaging such as a pressure plate, and the frictional engaging device is engaged. At this time, in general, running conditions such as throttle pressure and vehicle speed, etc. are detected by sensors, and the detected information is sent to a control unit (CPU). Further, electrical signals are sent from the control unit (CPU) to each solenoid valve controlling the hydraulic control mechanism and the hydraulic switching mechanism, and the solenoid valves are controlled the electrical signals.
Consequently, in the case where hydraulic pressure is used, electrical signals which are control signals are inevitably converted into hydraulic pressure. Due to this, it is difficult to conduct precise control and make a simple structure because of oil leakage and response delay. As a result, reliability is lowered.
In addition to the above, an oil pump is required having a power source for producing hydraulic pessure. In general, though an output from an engine is used as a power source for the oil pump, the engine does not work only for the oil pump, so the engine output has a wide range of rotational speeds, and accordingly the power source of the oil pump has to bear a great amount of loss, and additional efforts are required to design the manner of power takeout to power the pump. On the other hand, where an electric motor is used as an exclusive power source for the oil pump. However, in this case, because electric energy which is easy to control has to be converted into hydraulic pressure, efficiency has to be sacrificed.
Particularly, in the case where one function attained by one frictional engaging device is released and at the same time another frictional engaging device is engaged, both frictional engaging devices are controlled by respective hydraulic actuators. Then controlling and synchronizing two frictional engaging devices whose motions are required to be in opposing relation requires a complicated hydraulic control mechanism, and an additional further complicated hydraulic control mechanism is required for safety purposes to prevent a locking of the transmission device caused by simultaneous engaging of two frictional engaging devices.